1. Field of the Invention
The present invention relates to an apparatus for countercurrent contacting of gas and fluidized solids, and particularly to a stripper for a fluid catalytic cracking process.
2. Background of the Art
Various chemical, petroleum refining and combustion processes require contacting fluidized solid particles with an up-flowing gas to achieve efficient mass and or heat transfer. Such systems could also be used to carry out chemical reaction by efficient countercurrent contacting of the phases. One such widely used application is in a very important process known as fluid catalytic cracking (“FCC”), which has been used in petroleum refining for many years for converting heavier (or high boiling) hydrocarbon compounds to more valuable lighter (or low boiling) components. There are many different configurations of FCC units but all are essentially comprised of the same functional pieces of equipment. Generally, FCC units include a riser, which is a vertical pipe reactor in which the cracking reaction takes place. The hydrocarbon feed, generally in liquid form, is introduced at the bottom of the riser where it is contacted with hot regenerated catalyst. The catalyst being in the form of a fine powder, readily mixes with the oil and gives up heat to vaporize the oil and sustain the cracking reaction. Both the catalyst and hydrocarbon flow upward through the riser. It is advantageous to maximize the ratio of catalyst to oil for improved conversion in the riser. The residence time in the riser is typically less than about 10 seconds. A riser termination device at the top of the riser separates the reaction products (in a vapor phase) from the catalyst. The product vapors exit the FCC unit and are sent on to further processing. The catalyst particles, which are covered with coke from the cracking reaction, are sent to a regenerator wherein the coke is burned off in a stream of oxidizing gas (usually air). The catalyst coming from an atmosphere of hydrocarbon vapors will have hydrocarbons in the pores of the catalyst and between the catalyst particles. Before the catalyst can be sent to the regenerator it is advantageous to first strip the catalyst of entrained hydrocarbons or the hydrocarbons located in the pores of the catalyst. This is usually accomplished by contacting the downflow of catalyst particles with an upflow of steam. Efficiency of the stripping unit is important as unrecovered/unremoved hydrocarbons represent a loss of valuable product. The unrecovered/unremoved hydrocarbons also put an additional load on the regenerator in terms of burning a greater mass of hydrocarbons which would require additional quantity of oxidizing or combustion air. The regenerator would also operate at a higher temperature which would lower the catalyst to oil ratio and degrade the unit performance with regard to operating flexibility and the yield of valuable products. In addition, the hydrogen rich entrained hydrocarbons would yield higher steam partial pressure in the regenerator which would increase catalyst deactivation.
Referring to FIG. 1 a schematic view of a prior art FCC unit is shown. A high boiling feed such as gas oil or vacuum gas oil, or even heavier feed, is added to a riser reactor 6 through feed injection ports 2 where it mixes with the regenerated catalyst particles. The cracking reaction is completed in the riser and the cracked hydrocarbons and spent catalyst are diverted by elbow 10 through cyclones 12 which separate most of the spent catalyst from the product. Vapor from cyclone 12 along with the remaining catalyst particles is directly sent to cyclone 16 which removes most of the remaining catalyst. A small amount of cracked hydrocarbon vapor is carried down the diplegs of the cyclones along with the separated catalyst.
Spent catalyst is discharged down from the dipleg of cyclones 12 and 16 into catalyst stripper 8, where one or more stages of steam stripping occur. Stripping steam is injected via lines 19 and 21 either at the bottom of the vessel or at some intermediate point between the top and bottom of the vessel. The stripped hydrocarbons and stripping steam pass upward into disengager 14 and are removed with the cracked products after passage through the special openings in cyclones 12. There are many other cyclonic and non-cyclonic arrangements and devices for separating catalyst and cracked hydrocarbon vapor at the outlet of the riser reactor. However, the purpose of these other devices and arrangements is the same, i.e., to separate the catalyst particles from the cracked hydrocarbon vapors.
Stripped catalyst is discharged down through standpipe 26 into the horizontal catalyst transfer line 27. The flow of catalyst is controlled by a valve 36. Air is injected via line 28 into the transfer line 27, which causes the spent catalyst to be transported to the regenerator by dilute phase transport through transfer line 27 and lift line 29. One skilled in the art would appreciated that there are other suitable methods of transferring catalyst which can alternatively be employed.
Catalyst is regenerated in regenerator 24 by contact with air added via air lines and an air grid distributor (not shown). A catalyst cooler 28 may be provided so that heat can be removed from the regenerator in certain operations when higher boiling feeds are processed.
Regenerated catalyst is withdrawn from the regenerator via duct 34 into through hopper 31, standpipe 33 and valve assembly 30 and transported through lateral line 32 into the base of the riser reactor. Flue gas and some entrained catalyst are discharged into a dilute phase region in the upper portion of regenerator 24. Catalyst entrained with the flue gas is separated from the flue gas in multiple stages of cyclones 4. Separated catalyst is returned to the catalyst bed in the regenerator through diplegs 35. The flue gas is discharged through plenum 20, cooled to recover the sensible heat and discharged into the atmosphere.
A typical FCC unit catalyst stripper uses circular, conical baffles to facilitate the contacting of the catalyst with the stripping steam. The conical baffles are usually deeply inclined so as to prevent catalyst from reposing on the baffle. FIG. 2 shows a typical prior known arrangement of baffles in a stripper 40 including outwardly slanted baffles 41 alternating with inwardly slanted baffles 42. The baffles 41 and 42 tend to laterally shift the downflow S of catalyst particles back and forth against the upflow G of stripping gas to increase solid-vapor contact and mass transfer. However, it has been found that as the catalyst mass flux is increased through the stripper, the efficiency of mass transfer (or hydrocarbon removal in the case of FCC strippers), is reduced. Beyond a certain point the efficiency can drop off very sharply. Accordingly, a method and apparatus for achieving high efficiency mass transfer between downflowing fluidized particles and upflowing gas is needed.